1. Field of the Invention
The present invention generally relates to saw guides for wood sawing with lubricating and cooling capabilities which include externally pressurized gas bearings.
2. Description of Prior Art
Multiple blade saw systems are widely used throughout the lumber industry. Most multiple blade saw systems use very thin circular saw blades. Thin saw blades use less power and provide higher yield. However, thin saw blades require saw guide systems to stabilize the blades during cutting and prevent buckling. Typically this is done by sandwiching each blade between two flat lubricated surfaces which act as bearings supporting the blade. These bearing surfaces usually consist of a low friction material supported by a metal block adjacent the cutting zone.
Saw guide systems normally include a means of lubricating and cooling both the saw guides and saw blades. Typically liquid lubricants are carried into the narrow space between saw guide and saw blade in the form of a mist created by forcing high pressure air through a venturi nozzle or the like. Other systems apply liquid lubricants and coolants directly into the space. Some systems are provided with dual sets of passages in each saw guide to provide both air-oil and air-water mists.
A problem with saw guide systems in general is the wear of bearing surfaces. Guides require frequent changing. Typically babbitt saw guides last only a few hundred hours.
Sawguides with gas bearings are not widely used in industry. However, the advantages of gas lubrication over liquid lubrication are well known. Due to the low viscosity of the gas lubricant, gas bearings have very low coefficients of friction. The stability of gas as lubricant allows gas bearings to operate in both high and low temperature environments where liquids would solidify, vaporize, cavitate, or decompose. Gas bearings operate with very small clearances and thus maintain close tolerances.
The disadvantages of gas bearings are also recognized. Prior art teaches that conventional gas bearings have a low unit load carrying capacity. Generally gas bearings are larger and operate with very thin films compared to their liquid lubricated counterparts. The thin films in gas bearings demand very close control of machining tolerances and surface finishes.
The unit load capacity of gas bearings is pressure dependant. High unit loads require high gas pressures. Gas bearings are often thought to be incapable of operating at unit loads achievable by liquid lubricated bearings. There are many analytical and experimental studies on the phenomena of supersonic pressure depression in the feeding region of externally pressurized gas bearings. The deductive conclusion of this teaching is that raising the inlet pressure on a particular gas bearing can have a negative effect on load capacity. However, in many such cases the internal shock surfaces arise primarily from the rapid expansion of the flow cross sectional area.
The supersonic gas bearing Miyake et al., U.S. Pat. No. 4,486,105, is based on a recognition of the advantage of deliberately varying the flow cross sectional area to regulate the character of the flow within the bearing clearance space. In this type of gas bearing the flow cross sectional area is varied along the flow path to form a smooth converging-diverging nozzle. Under the nominal design conditions, the flow enters the converging section at a subsonic velocity, gradually accelerates to sonic velocity at the at the end of the converging portion, and then continues to accelerate at supersonic velocities in the diverging section to the edge of the bearing. This type of bearing has a much higher unit load capacity than a conventional gas bearing. However, supersonic bearings typically consume large quantities of gas. This is due primarily to the relatively large clearance gap utilized to make the area change effects dominate relative to other effects, such as flow friction and heat transfer, and produce the smooth flow transitions from subsonic to supersonic velocities within the bearing clearance space.
Heretofore, prior art has failed to appreciate the advantages of controlling the character of the flow with cross sectional area control in conjunction with frictional effects within a gas bearing. It is possible to have both high pressure, low gas consumption, and subsonic flow by controlling flow cross sectional area in combination with significant friction. Specifically, it is not necessary have a region of supersonic flow, as in the supersonic bearing, to obtain high unit load capacity. Additionally, prior art does not appreciate the advantages of maintaining a Mach 1.0, or sonic exit conditions at the exterior edge of the bearing surface.
Prior art includes a number of considerations of compressible flow effects which are relevant to a specific application. These include air hammer instability and lock up which are sometimes related to each other. Lock up can occur when the gas supply pressure multiplied by the area of the recess is less than the load. Avoiding lock up is crucial in many situations such as opposed pad thrust bearings used in sawguides.
Heretofore, the closest prior art to present invention is Kordyban, U.S. Pat. No. 4,136,513, a floating pad saw guide. Air is used as the primary lubricant. However, the asymmetry in the design is indicative of an approach using low pressures and low velocities across the bearing surfaces relative to saw tip speed. The single small circular supply hole and a uniform clearance space gap preclude establishing a shock surface at the exterior edge of the bearing surface and a gradually increasing velocity along the flow path.
Gas bearings with small clearances are often limited by the expense of manufacturing surfaces of the required tolerances. The use of self aligning bearing surfaces can greatly reduce the necessity for expensive machining while providing close tolerances. However, the advantages of compliant support in controlling the response and cross sectional flow area of gas bearings are not appreciated by prior art.